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8/13/2019 Jma Slides Halmhuse
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Straw Bale Houses -
design and material properties
Jrgen Munch-Andersen
Birte Mller Andersen
Danish Building and Urban Research
(Statens Byggeforskningsinstitut)
Research Project
Aim: to demonstrate how straw bale houses can be buildas durable and well performing structures
Two part project:
1. Material properties
2. Design guideline
Supported by the Danish Energy Agency, special
programme for environmentally friendly insulation
Material Properties
Thermal insulation properties
Moisture transport and condensation
Settlements
Fire properties
Sound insulation
Working environment
Heat insulationTwo test types
1. Thermal conductivity (-value) from standard tests
2. Direct U-value from guarded hot box test
Thermal conductivityMaterial property depending on straw type, straw direction
and density. Mineral wool has ~ 0,035 W / mK
U-valueProperty of wall, depending mainly on thickness and .
200 mm mineral wool gives U ~ 0,18 W / m2K
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U-values from -values
Measured for 385 mm straw parallel to heat flow, 75 kg / m3
with 80 mm clay plaster: U = 0.21 W / m2K (excl. air filmresistance)
Calculating from -measurement:
Heat flow parallel to straw, 75 kg / m3: = 0.057 W / mK
Clay plaster, assumed: = 0.8 W / mK
0,39 m straw + 0,08 m plaster => U = 0,15 W / m2K
Direct measurement about 50% higher than calculated!
Reasons for difference in U-value
Convection: American calculations by CFD indicates
serious effect, increase U from 0.15 to 0.17
Intrusion of plaster into straw might reduce effective
thickness of straw by some 2 x 10 mm, increase U by
0.01 to 0,18
Intrusion of plaster in cavities at rounded corners willfurther decrease effective thickness of straw. Assuming
an effective average of 2 x 25 mm changes U to 0.19
Difference not explained fully
Conclusions, U-value
Danish standard bales, 450 mm in straw direction and 360mm perpendicular to straw direction
Design U-values can be taken as:
Walls with straw parallel to heat flow:
U = 0,18 W / m2
K
Low pitch roofs with straw perpendicular to heat flow:
U = 0,18 W / m2K (low pitch prevents convection caused by air flow)
Moisture and condensationTwo test types
1. Water vapour resistance
of plaster and straw
2. Hot-cold box
Water vapour resistanceMaterial property for resistance against water vapour
penetration, Z-value. A vapour barrier has Z ~ 100
Amount of water transport depends on Z and difference inpartial water vapour pressure
Z, GPa s m2/kgSample For 40 mm layer
Wet Dry
1 Clay plaster 1.6 2.42 Clay plaster with 5 layer of whitening 1.7 2.43 Clay plaster painted with linseed oil 2.6 3.54 Clay plaster mixed with linseed oil, 1% 2.5 3.15 Lime, coarse (0-4 mm) 2.1 2.26 Lime, fine (0-2 mm) 3.0 -7 Clay plaster mixed with chopped paper, 5% 1.8 -8 Clay plaster mixed with chopped straw, 30% 1.8 -9 Clay plaster mixed with cow manure, 25% 1.5 -10 Clay pl aster painted with si licat e paint sys tem, four layers 2.5 -
Hot-cold boxSimulation of winter conditions, 0C outside, 25 C inside
Clay plaster inside, clay and lime plaster outside
0 50 100 150 200 250 300 350 400Days
-10
0
10
20
30Temperature / oC
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0 50 100 150 200 250 300 350 400Days
8
10
12
14
16
18
20
22Moisture content / pct.
Air, warm side
Air, cold side
Behind clay stucco
Behind lime stucco
Period Warm side Cold side Difference
Days T, C ps, Pa RH, pct. pv, Pa T, C ps, Pa RH, pct. pv, Pa pv, Pa
73-102 24,8 3132 46,3 1450 -1,8 526 85,0 447 1003
103-108 25,0 3169 48,0 1521 -1,5 540 83,9 453 1068
110-134 25,3 3245 45,1 1463 1,7 392 88,8 614 822
229-259 25,0 3169 44,0 1394 -1,1 558 83,5 466 928
260-285 24,9 3151 44,8 1412 -1,0 562 84,7 476 936
286-316 24,6 3095 43,5 1346 -1,0 562 86,0 483 863
349-379 24,8 3132 43,2 1353 -1,4 544 85,2 463 890
Difference in partial water vapour pressure
Settlements of plastered wallsTwo walls, Relative humidity varied between 50 and 80 %
1. Non-load bearing, normal straw bales (75 kg/m3)
2. Load bearing, 5 kN/m, big bales (125 kg/m3)
Measured settlements
Fire tests
Two test types:
1. Resistance
('wall')
BD 30 ~ EI 30
2. Protection
('ceiling')
kl A ~ B-s1,d0
Resistance, after test
Fire put out after 30 min, most likely that EI 60 is fulfilled
Plaster protected sides of posts well
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Protection, after test
Plaster remained on straw for 10 min
Protruding straw burned - ok
Sound reduction
Internal plastered straw bale
wall measured on-site
R'w= 52 dB
Requirement for terracedhousing: 55 dB
Working environment High level of organic fine
particle dust, severely
affected by in-door work
Low level of fungi
thanks to fresh and yellowstraw
Mussel shells as floor slab
insulation
Thermal conductivity and capillary suction measured forwhole shells, crushed shells and the coarse fraction of the
crushed shells.
~ 0,12 W / mK
Capillary suction height< 25 mm
Design guidelines Windows
and daylight
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Roofs, example 1
Roof felt on boards
Trussed beam
Ventilated cavity
Plaster
Straw bale
19 mm boards at 300 mm2 x 13 mm plasterboard
Roofs, example 2
Roof cladding / battens
Foil membrane
Secondary beams
Ventilated cavity
Plaster
Straw bale
50 mm Class A insulation
Boards, notched
Primary beams
Connection btw. beams
Roofs, example 3
'Green roof'
Drainage membrane on
plywood board
Secondary beams
Ventilated cavity
Plaster
Straw bale
Plaster
Primary beams
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Roofs, summary
Fire requirements fulfilled in3 different ways
Plaster on upper side
improves fire safety andheat insulation
Extra insulation advisable
Walls, example 1
Plaster
Straw bale
Plaster
Timber pole, free standing
Walls, example 2
Plaster
Straw bale
Plaster
Timber pole, partly build in
Walls, example 3
Plaster
Straw bale
Plaster
Timber pole, build in
Walls, example 4
Timber cladding
Ventilated cavity
Plaster
Straw bale
Plaster
Timber truss beam
Structure 1
Ventilation of roof
Load distribution on wall
Head beam allows openings
Point foundation of poles
LWC-block, preventingcapillary suction
Membrane to protect againstradon intrusion
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Structure 2
Ventilation of roof
Head beam allows closespacing of roof trusses
Line foundation
Moisture barrier
LWC-block with insulation
Membrane to protect againstradon intrusion
Structure 3
Foil membrane to gutter
Ventilation of roofFrame structure with cladding
Allow every 3rd or 4th leg to becut away
Line foundation
Frame only supported at inside
Moisture barrier
LWC-block with insulation
No membrane to protectagainst radon intrusion
Conclusions
Straw bale houses can fulfil all requirements to houses
especially fire regulations
Detailing very important to avoid damage fromwater vapour, moisture and rain
Inside must be airtight
External cladding by eg. boards reduces sensitivity tomaintenance
Heat insulation only fair
Methods to reduce convection should be studied